It is well-known that various industrial evaporation apparatuses are used to remove volatile components and/or low boiling point components by evaporation from a liquid and thus concentrating the liquid. For example, Kagaku Kogyo Binran (“Chemical Engineers' Handbook”)6th Edition (edited by the Society of Chemical Engineers, Japan, 1999: Non-Patent Document 1) (see pages 403-405) describes the following types of industrial evaporation apparatuses: a submerged combustion type, a natural circulation immersed tube type, a natural circulation horizontal tube type, a vertical short tube type, a vertical long tube climbing film type, a horizontal tube falling film type, a vertical long tube falling film type, a forced circulation horizontal tube type, a forced circulation vertical tube type, coil type, an agitated film type, a centrifugal thin film type, a plate type, and a flash evaporation type. Of these industrial evaporation apparatuses, for a system in which the liquid is made to flow from the top downward, there are the horizontal tube falling film type, the vertical long tube falling film type, the forced circulation horizontal tube type, the forced circulation vertical tube type, and the agitated film type. With the exception of the agitated film type, all of these are of the same form as a multi-tube cylindrical heat exchanger. For the agitated film type, scrapers are rotated over a cylindrical or conical inner surface which is heated through an outer heat source, thus scraping a liquid film off from the heat transfer surface as the liquid film is formed thereon, whereby the liquid is concentrated through evaporation while making the evaporation uniform and promoting heat transfer. Moreover, with the horizontal tube falling film type or the forced circulation horizontal tube type, which are of the heat exchanger type, the liquid is concentrated through evaporation while being made to flow in the form of a liquid film over external surfaces of horizontally installed tubes; the tubes themselves are heated through a heating medium such as steam flowing therethrough.
In terms of the flow of the liquid, the industrial evaporation apparatuses closest to the type of the present invention are the vertical long tube falling film type and the forced circulation vertical tube type, but these are of a type in which the liquid is concentrated through evaporation while descending in the form of a film through vertically installed tubes, as opposed to the system of the present invention in which the liquid is made to flow down along external surfaces of guides, and moreover the tubes through which the liquid flows down are themselves heated by a heating medium that flows over the outside (trunk portion) of each of the tubes, unlike in the present invention in which the guides do not themselves have a heat source.
Moreover, strand evaporators in which a melt of a polymer or the like is extruded in the form of strands or filaments from a perforated plate into an evaporation zone, and the polymer melt is concentrated through evaporation while falling down freely are also known (See Patent Document 1: U.S. Pat. No. 3,110,547; Patent Document 2: Japanese Patent Publication No. 30-2164). However, there are drawbacks with such a strand evaporator, for example, because the liquid to be concentrated is allowed to fall down freely, the residence time in the evaporation zone is short, and hence the evaporating efficiency is poor, and moreover the strands or filaments sway sideways in the evaporation zone and are thus liable to fuse to one another, and hence continuous stable operation is difficult. Apparatuses in which a polyamide, a polyester or the like is produced while making a monomer mixture or prepolymer flow down along linear supports have also been proposed (See, for example, Patent Document 3: U.S. Pat. No. 3,044,993; Patent Document 4: Japanese Patent Publication No. 48-8355, Patent Document 5: Japanese Patent Application Laid-open No. 53-17569; Patent Document 6: Japanese Patent Application Laid-open No. 60-44527; Patent Document 7: Japanese Patent Application Laid-open No. 61-207429). Furthermore, an apparatus in which wire loops are installed in an evaporation zone, and a high-viscosity polymer melt is concentrated through evaporation or has gas removed therefrom while being made to flow down along the wire loops has been proposed, and it has been stated that such an apparatus can be advantageously used for subjecting a polycarbonate solution or melt in particular to concentration through evaporation or gas removal (See Patent Document 8: International Publication WO 2002/051606); this method is substantially the same as a method and apparatus already proposed by the present inventors in which a polycarbonate is produced by polymerizing a molten polymerization starting material while making the molten polymerization starting material flow down along wires or perforated sheet-shaped guides (See Patent Document 9: International Publication WO 99/36457). However, with such an apparatus having the wires or the perforated sheet-shaped guides, there have been no specific disclosures or suggestions relating to the scale or use of an apparatus which can carry out the evaporation operation stably for a prolonged period of time on an industrial scale such that not less than 1 ton/hr of the liquid is subjected to the evaporation.
Furthermore, with evaporation apparatuses known so far, there has been no description of means for avoiding degeneration such as discoloration, gelation, crosslinking, production of an ultra-high molecular weight, solidification, scorching or carbonization due to some of the liquid residing in the apparatus for a prolonged period of time. In particular, in the case of an apparatus for evaporating lower boiling point material from a liquid having a relatively high viscosity, it has been revealed for the first time through carrying out prolonged continuous operation that such degeneration occurs, then the degenerated material progressively or suddenly becomes mixed in the concentrated liquid, bringing about problems of discoloration and contamination with solid foreign material, which are fatal for the concentrated product such as polymer etc.
However, it is clear that with the evaporation apparatuses known so far, no consideration whatsoever has been given to such degeneration upon prolonged operation. For example, according to FIG. 1 in Patent Document 5, which relates to an evaporation apparatus that uses linear supports, it is clear that there is much so-called “dead space” where the polymer resides for a prolonged period of time and is thus heated between a hole portion of a perforated plate and an internal sidewall surface of a hollow body, and no means whatsoever has been adopted for reducing this “dead space”. Accordingly, the high-viscosity material fed in from a high-viscosity material feeding port (7) resides for a prolonged period of time in the “dead space” between the perforated plate (3) and the internal sidewall surface of the hollow body, and hence the above problems occur inevitably. Moreover, even in recently proposed by Patent Document 8, no means whatsoever has been adopted for reducing “dead space” in a distributor pipe 3 (FIG. 2), or a feeding zone from which feeding is carried out onto a perforated plate (FIG. 3a).